This invention relates to a modulus of elasticity testing apparatus and associated methods for same. More particularly the invention is related to a method and apparatus for testing compliant materials for determining the modulus of elasticity of such materials used in printed circuit boards.
The modulus of elasticity is defined as the ratio of the unit stress to the unit deformation of a structural material and basically is a constant as long as the unit stress is below the proportional limit. The shearing modulus of elasticity is frequently called the modulus of rigidity and there are other moduli which are determinative of elasticity measurements.
As is known, elasticity is a property whereby a body when deformed automatically recovers its normal configuration as the deforming forces are removed. Each of the several types is probably due to the action of the intermolecular forces which are in equilibrium only for certain configurations. Deformation, or more briefly, strain, is of various kinds and in each case it is the measure of a certain abstract ratio. In any event, the modulus of elasticity has been utilized to determine the characteristics of various materials. Measurement equipment for determining the modulus of elasticity is utilized in the prior art and is utilized for a wide variety of materials.
As one can ascertain, various tables exist for different materials which give the modulus of elasticity for the material. In regard to such measurements, one is normally concerned with a quantity defined as Poisson's ratio which is equal to the relative lateral contraction divided by the relative longitudinal extension. Poisson's ratio for various materials is known and has been defined.
As indicated, the prior art utilized various modulus measuring mechanisms which are essentially employed to measure the stress relationship of materials, and from the stress strain curve the modulus of the material can be derived. In any event, in order to use such equipment, the material to be measured must be prepared into a standard sample configuration. This standard sample configuration which is of a particular length, width and thickness is utilized in conjunction with applied forces to determine the various moduli.
The methods used in the prior art employ a tensile force measurement rather than an angular displacement. Thus such prior art techniques require time consuming operations in regard to the preparation of the standard sample. The sample then may be placed in a tensile machine as between a pair of jaw vises and is stressed at given forces in order to measure the tensile stress or strength. These measurements essentially depict deformation of the material and hence the various moduli can be derived by suitable equations.
In any event, the theory of elasticity and the various parameters involved in regard to elasticity have been the subject matter of many articles and many textbooks. See for example, a textbook entitled Introduction to Elasticity, published by Holt, Rinehardt and Winston, New York 1964 by G. Nadeau. See also a text entitled Theoretical Elasticity and Plasticity, published by Thames and Hudson Company of London, England 1959 by D. E. R. Godfrey. It is indicated that the present invention is particularly directed for accepting compliant material for use with surface mounted devices as in coated printed circuit boards.
Essentially, as one can ascertain, in the semiconductor art surface mounted devices such as surface mounting connectors and components are widely employed, and there have been many articles written about the advantages of surface mounting. The surface mounted component is usually mounted on a printed circuit board and is positioned on the board by means of various solder joints which connect the surface mounted component to the board at various terminal areas.
When utilizing such printed circuit boards in conjunction with surface mounted components and solder joints, one experiences a particular problem in regard to temperature changes. The problem is that the surface mounted components have widely different temperature coefficients from those of the printed circuit board. The printed circuit boards which are typically made from a glass epoxy material exhibit larger expansion coefficients than the surface mounted components.
Hence as one can ascertain, during temperature cycling of the assembly, the printed circuit board tends to expand at a greater rate than the surface mounted components. This causes unusual stresses in the solder joints and by constant temperature cycling one can actually fatigue the solder joints, resulting in a complete failure of the printed circuit module.
In any event, in order to circumvent such temperature problems, various manufacturers have employed coatings sometimes referred to as a "butter coat". These coatings constitute a relatively thin film of a compliant or elastomeric material such as a rubber type material which is coated over the board. The thickness of such layers is between 5/1000 to 10/1000 of an inch. The compliant coating acts as a shock absorber and essentially functions to dissipate the energy would otherwise be coupled directly to the solder joints. In regard to this technique, the use of the elastomer serves to protect the board during large temperature cycling operations.
It is a problem in regard to the particular user of such an assembly to determine what the modulus of elasticity is so one can ascertain that the boards will operate over the required temperature range. It is desirable that the composite structure which consists of the printed circuit board and the compliant layer or elastomeric layer is capable of matching and providing proper thermal expansion characteristics when utilized in conjunction with a surface mounted chip carrier or a component circuit board.
As indicated, the present technique for accepting compliant material for use with surface mounted devices is performed by thermal cycling test coupons. A test coupon is a test device which is exactly the same size and thickness as the resultant printed circuit board. This method is extremely costly and takes several weeks to be completed before the compliant material is accepted from the vendor. The exact nature of the particular problem and the solution thereof will be subsequently explained. Except, suffice it to say that there is a need to develop acceptance methods for complaint materials based on the material's physical properties. In addition a method and apparatus was needed to correlate thermal cycles to failure with physical material properties measured at incoming acceptance for each received lot of material.
The method and apparatus also will serve to eliminate making and preparation of complicated sample forms in regard to length, width and thickness, as will be further explained.
It is therefore an object of the present invention to provide a modulus of elasticity tester apparatus for compliant materials.
It is a further object of this invention to provide a modulus of elasticity tester for enriched resin compliant layer substrates employed in surface mounted printed circuit board applications.
It is a further object of this invention to provide a method for measuring the modulus of elasticity in compliant layer materials.
It is a further object of the present invention to provide a modulus of elasticity measurement apparatus and method for measuring the modulus of elasticity by responding to small angular deflections of the compliant layer without the necessity of processing material into a printed circuit board test coupon as for example implemented by the prior art.